Over the last 20 years geopolymerization has emerged as a potential alternative to Portland cement grouts for applications such as the immobilization of inorganic hazardous waste and, more recently, of radioactive waste. The application of geopolymer stabilization retains all of the simplicity of Portland cement grouting and is a simple one of mixing a waste stream with readily available, inexpensive, inorganic, non-flammable materials. Portland cement grouting, generally, might include the use of components such as fumed silica (silicon dioxide, amorphous fumed), meta-kaolin, fly ash, ordinary Portland cement, and others, depending on which compounds are not supplied by the waste in large enough quantities to make a stable waste form.
Geopolymeric materials or geopolymers are also referred to as alkali-activated aluminosilicate binders. These materials can be best described as equivalents of certain synthetic and natural zeolites, although the distinctive zeolite crystal structures are absent. Further, geopolymers are amorphous to X-rays.
The synthesis temperatures required for the formation of geopolymers typically range between 20° C. and 80° C. Structural integrity and mechanical strength are attained in a few hours, depending on the conditions of synthesis. Geopolymer waste forms lend themselves particularly to immobilization of waste streams with high alkali concentrations and acid waste streams require neutralization with sodium or potassium hydroxide. As with grouting, for optimum waste loadings, very dilute waste streams may require evaporation prior to treatment to remove the excess water.
In the radioactive waste processing industry it is desirable to engage in solidification, stabilization, and immobilization of radioactive and hazardous waste to minimize and, preferably prevent the potentially deleterious effects of these wastes on the environment. Solidification and stabilization technology is a treatment that is used to prevent or slow the release of harmful materials, such as chemicals or radioactive waste from contaminated soil, sludge, sediment, or other materials.
For example, cement-based solidification and stabilization is preformed by mixing Portland cement into the contaminated media, such as soil, sludge, or sediment to help make the waste safe for land disposal. As applied to radioactive waste, solidification and stabilization, although providing some additional radiation shielding, is principally used for physical immobilization of radioactive material. Immobilization of the radioactive material prevents release of those materials into the environment. Over time the level of radioactivity emitted from the immobilized radionuclides reduces itself through a process of radioactive decay. Therefore, solidification and stabilization allows for the contaminated material to be safely stored until radioactive decay reduces the level of radiation emitted from the treated material to an acceptable level.
When performing stabilization, solidification or immobilization, the resulting materials produced by these processes are referred to as “waste forms.” The most chemically durable materials for this purpose are typically formed at high temperatures and include glass waste forms produced by vitrification and waste forms produced by various ceramic processes. However, such “thermal” processes have associated disadvantages that include relatively high costs and the tendency to volatilize certain radioactive and/or hazardous waste materials or species. As a result, off-gas treatment systems are typically required to decontaminate the gasses that are produced in such processes before the gasses can be safely released into the environment. These off-gas treatment systems produce secondary waste streams that then require treatment and, ultimately, stabilization in yet another waste form. For all of these reasons, there is a need for low-temperature waste solidification processes that are better able to retain volatile radioactive and/or hazardous species and which do not require off-gas treatment systems or require greatly simplified off-gas treatment systems, all of which can potentially result in significant cost savings. Current, low-temperature non-thermal processes systems produce material that are less chemically durable than those produced from the thermal solidification processes. Consequently, there is a need for low-temperature processes that produce waste forms with enhanced chemical durability.
Further, certain types of radioactive and hazardous wastes contain organic compounds. Since these compounds, or reactions products from the decomposition of these compounds, can be hazardous to the environment, it is also desirable to immobilize these compounds in the waste form that results from treatment of the waste. A further aspect of the present invention is to effect the immobilization of such compounds in the waste form and to reduce the extent to which such compounds are degraded to produce more hazardous or more mobile compounds, or both, either during the treatment process or in the waste form itself, or both.
Herein, we define “low-temperature” and “non-thermal” processes to be processes that operate below approximately 150° C.
This invention produces a waste form that is a type of zeolitic, alkali aluminosilicate material that is based on geopolymer chemistry. However, the process of the present invention also include novel features that serve to significantly enhance the chemical durability of the waste form over the prior art and, more particularly, serve to significantly reduce the leachability of radioactive iodine and technetium from the waste form.
Radioactive iodine and technetium, and more specifically 129I and 99Tc, are among the major risk contributors in the environment because of their long half-lives, high mobility, and bioactivity. Radioactive iodine is a particular hazard to humans because iodine is an essential element in human diet to avoid thyroid-deficiency disease. The human body selectively extracts iodine from food, water and air, storing it mainly in the thyroid gland. Since radioactive and non-radioactive iodine are identical from a chemical viewpoint, humans and other living organisms are unable to differentiate one form of iodine from another. Studies have shown that radioactive iodine can cause fatal thyroid cancer. Therefore, the present invention can reduce the risk of storing radioactive waste that contains harmful elements such as 129I.
Further, radioactive iodine and technetium are volatile at higher temperatures and thermal processes do not adequately retain these elements in the resulting waste forms. As such, these elements can be present in significant amounts in the secondary waste streams generated from thermal processes. The low-temperature process of this invention minimizes volatilization and secondary waste production; provides a waste form with superior retention of technetium and iodine as well as heavy metals and other radioactive and/or hazardous species; and is chemically well matched to handle various types of highly alkaline salt waste streams.